In most of known navigation systems of the above said type, the radar beacon responds at a frequency which is near the frequency of the received radar pulse and the response signal is presented on a display superposed on the normal radar image. An important problem is that powerful ground return, rain- and sea clutter can completely or partially mask the signal of the transponder particularly if the transponder is situated a short distance away. The fact that the signal of the radar beacon is superposed on the normal radar display may also allow the response signal hide other echos of interest which is also considered as a drawback. For civil-maritime radar communication there are two allotted frequency bands, the so-called 3 cm. band or X-band between 9300 and 9500 MHz and the so-called 10 cm. band or S-band between 2900 and 3100 MHz.
In the case of so-called permanent frequency radar beacons, these transmit, when detecting a radar pulse, a response pulse with a permanent frequency, for example, in the lower part of said frequency band. The vessel-borne radar can be readjusted for receiving such signal and in this way there is made an attempt to distinguish the response signal from disturbing signals possibly arising in consequence of the transmitted radar signal. However, in a vessel-borne radar with frequencies near the response frequency, disturbing signals also arise at the response frequency in the form of ground- and sea clutter. The reason is that the transmitted radar signal and consequently the clutter is broad in frequency and thus the clutter will be situated within the frequency range of the transponder. Furthermore, the radar receiver has a finite attenuation for the clutter frequency even when the receiver is adjusted to the response frequency. For these radars also an effective receiver selection is necessary in order to prevent racon signals at from resulting in interference on the normal radar display. Another drawback, is when the radar and the radar beacon are transmitting with different frequencies, is that the majority of radar aerials existing today are of the "end fed slotted waveguide" type which has the characteristics that the direction of the aerial beam is frequency dependent and varies at 0.8.degree.-1.0.degree./100 MHz. This can result in a deviation of bearing to the radar beacon and furthermore in a strongly reduced range of the same.
Another approach is the so-called frequency offset method in which the response transmitter answers with a definite frequency which is for example 50 MHz below the frequency transmitted from the radar. Also in this case the interference signal arises for the same reasons as mentioned above in connection with permanent frequency radar beacons. Reference is made, for example, Conceptual Radar Piloting Techniques Using Radar Beacon (Racon) Technology and Other Advanced Marine Radar Technology, E. F. Greneker and J. E. Metthews, Georgia Institute of Technology, April 1981 and Fixed Frequency Racon, a performance analysis prepared for the Swedish Administration of Shipping and Navigation, by Bo Morwing and P. O. Gustavsson.